Further Investigation of Standoff Screws Used in Composite Joists

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The purpose of this study is to further evaluate the performance of the 5/16 in. diameter Elco Grade 8 standoff screw as a mechanical shear connector in composite joists. Standoff screws are being investigated as viable an alternative to welded shear studs in short span composite joists. The data and results obtained from 59 pushout tests performed on the 5/16 in. diameter Elco Grade 8 standoff screw are presented. The test parameters investigated in this study include: standoff screw height, quantity of standoff screws per deck rib, standoff screw position, slab depth, base angle thickness, deck type, and amount of transverse reinforcement.
Test results gathered in this study are used in conjunction with selected test data from research performed by Alander (1998). This combined test data is used in determining the validity of existing predictive equations for the shear strength of the 5/16 in. diameter Elco Grade 8 standoff screw. The influence of various independent variables on shear strength is investigated for all screw densities tested. The performance of the 5/16 in. diameter Elco Grade 8 standoff screw in solid slab applications is also investigated. Proposed predictive equations for the shear strength of the 5/16 in. diameter Elco Grade 8 standoff screw based on screw-related failure modes, concrete cone failures and longitudinal splitting, are presented.